Solar Thermochemical Water Splitting Research Articles

Advances in Functional Ceramics for Water Splitting: A Comprehensive Review

The global demand for sustainable energy sources has led to extensive research regarding (green) hydrogen production technologies, with water splitting emerging as a promising avenue. In the near future the calculated hydrogen demand is expected to be 2.3 Gt per year. For green hydrogen production, 1.5 ppm of Earth’s freshwater, or 30 ppb of saltwater, is required each year, which is less than that currently consumed by fossil fuel-based energy. Functional ceramics, known for their stability and tunable properties, have garnered attention in the field of water splitting. This review provides an in-depth analysis of recent advancements in functional ceramics for water splitting, addressing key mechanisms, challenges, and prospects. Theoretical aspects, including electronic structure and crystallography, are explored to understand the catalytic behavior of these materials. Hematite photoanodes, vital for solar-driven water splitting, are discussed alongside strategies to enhance their performance, such as heterojunction structures and cocatalyst integration. Compositionally complex perovskite oxides and high-entropy alloys/ceramics are investigated for their potential for use in solar thermochemical water splitting, highlighting innovative approaches and challenges. Further exploration encompasses inorganic materials like metal oxides, molybdates, and rare earth compounds, revealing their catalytic activity and potential for water-splitting applications. Despite progress, challenges persist, indicating the need for continued research in the fields of material design and synthesis to advance sustainable hydrogen production.

Investigation on the activity of Ni doped Ce0.8Zr0.2O2 for solar thermochemical water splitting combined with partial oxidation of methane

The incredible potential of metal oxide-based two-step solar-driven thermochemical water splitting technology lies in its ability to harness limitless solar energy to produce clean and renewable hydrogen fuel. However, achieving high redox kinetics and lowering the reaction temperature remain the primary challenges in hydrogen production. The CeO2–ZrO2 system is a well-known oxygen carrier for thermochemical water splitting on account of its high temperature stability, quick kinetics, and redox cyclability. While recent studies have explored the combination of methane partial oxidation with metal oxide reduction, the reduction effect of cerium-zirconium oxide is relatively low, and reactivity still needs improvement. The nickel-doped system exhibits good redox performance and is regenerative. In this study, different mass percentages nickel-loaded oxygen carriers mNi/Ce0.8Zr0.2O2 (m = 0, 1, 3, 5, 7, 9) were prepared using the impregnation method. Redox properties of oxygen carriers were investigated in a fixed bed reactor and various characterizations were performed. To assess the durability of mNi/Ce0.8Zr0.2O2, 60 long-cycle experiments were also conducted. The results indicate that the lattice oxygen in mNi/Ce0.8Zr0.2O2 exhibits remarkable selectivity for the partial oxidation of CH4 at temperatures ranging from 880 to 925 °C. The best reactivity performance was achieved with a nickel loading ratio of 5 wt%, and after 60 consecutive redox cycles, 5Ni/Ce0.8Zr0.2O2 demonstrates remarkable robustness, maintaining its redox activity.

(Invited) Hydrogen Consortium: Technical Progress on Renewable Hydrogen Production R&D

HydroGEN Energy Materials Network (EMN) is an U.S. Department of Energy (DOE) EERE Hydrogen and Fuel Cell Technologies Office (HFTO)-funded consortium that aims to accelerate the discovery and development of advanced water splitting materials (AWSM) for clean, low-cost hydrogen production. This is in line with the H2@Scale initiative, with the goal to meet U.S. DOE’s Hydrogen Shot production cost target of $1/kg H2 within 1 decade (1 1 1). Materials innovations are key to enhancing performance, durability, and cost of hydrogen generation technologies. Large scale, low cost hydrogen from diverse domestic resources can enable an economically competitive and environmentally beneficial future energy system across multiple sectors.HydroGEN is focused on low technology readiness level AWS technologies, including low- (alkaline exchanged membrane electrolysis) and high-temperature electrolysis (proton-conducting solid oxide electrolysis), photoelectrochecmical (PEC) and solar thermochemical (STCH) water splitting. This presentation will provide an overview of the HydroGEN EMN and technical highlights of a few lab-led and FOA-awarded R&D projects. HydroGEN continues to grow its community of industry, university, and national laboratories, forming a national innovation ecosystem focused on renewable hydrogen production.

High-entropy spinel oxide (Fe0.2Mg0.2Mn0.1Al0.3Cr0.2)3O4 as a highly active and stable redox material for methane driven solar thermochemical water splitting

Solar thermochemical H2O splitting has attained wide attention as a promising solution for hydrogen production. However, it remains a daunting challenge to produce hydrogen with high kinetics and stability for the oxide materials at lower temperature because of insufficient active sites for H2O activation and splitting. Herein, we report a high-entropy spinel oxide that consists of five cations (HEO) and can highly-effectively thermochemically split H2O to H2 with ultrahigh production rate and productivity of 182.9 mL min-1 g-1 and 68.5 mL g-1, respectively, several times higher than state-of-the-art materials via a two-step cycle process with methane driven reduction. High-entropy effect facilitated the preservation of single-phase structure even at large amount of oxygen converted (almost 70 mL g-1), which promoted the exsolution and stabilization of substantial Fe0 nanoparticles (20–30 nm vs. 100 nm) and their dissolution into spinel structure during redox process, resulting in extensive metal-oxide interfaces with metal-oxygen vacancy pairs responsible for the efficient and stable water splitting. Such findings provide a class of viable material with high configurational entropy for efficient hydrogen generation.

Technological Pathways to Produce Compressed and Highly Pure Hydrogen from Solar Power.

Hydrogen (H2 ) produced from renewables will have a growing impact on the global energy dynamics towards sustainable and carbon-neutral standards. The share of green H2 is still too low to meet the net-zero target, while the demand for high-quality hydrogen continues to rise. These factors amplify the need for economically viable H2 generation technologies. The present article aims at evaluating the existing technologies for high-quality H2 production based on solar energy. Technologies such as water electrolysis, photoelectrochemical and solar thermochemical water splitting, liquid metal reactors and plasma conversion utilize solar power directly or indirectly (as carbon-neutral electrons) and are reviewed from the perspective of their current development level, technical limitations and future potential.

Technologische Pfade für die Herstellung von komprimiertem und hochreinem Wasserstoff mit Hilfe von Sonnenenergie

AbstractWasserstoff (H2) aus erneuerbaren Energien wird einen wachsenden Einfluss auf die globale Energiedynamik hin zu nachhaltigen und kohlenstoffneutralen Standards haben. Der Anteil von grünem H2 ist noch zu gering, um das Netto‐Null‐Ziel zu erreichen, während die Nachfrage nach hochwertigem Wasserstoff weiter steigt. Diese Faktoren verstärken den Bedarf an wirtschaftlich tragfähigen H2‐Erzeugungstechnologien. Der vorliegende Artikel zielt darauf ab, die bestehenden Technologien zur Erzeugung von hochwertigem H2 auf der Grundlage von Solarenergie zu bewerten. Technologien wie die Wasserelektrolyse, die photoelektrochemische und die solare thermochemische Wasserspaltung, Flüssigmetallreaktoren und die Plasmakonversion nutzen die Sonnenenergie direkt oder indirekt (als kohlenstoffneutrale Elektronen) und werden im Hinblick auf ihren derzeitigen Entwicklungsstand, ihre technischen Grenzen und ihr Zukunftspotenzial untersucht.

Synthesis, Experimental, and Theoretical Study of Co3-x Ni x O4 Mixed Oxides: Potential Candidates for Hydrogen Production via Solar Redox Cycles.

Recently, green hydrogen production via solar thermochemical water splitting (STWS) as a clean and sustainable method is becoming a subject of interest to many researchers. Great efforts are being made to develop materials for STWS with suitable operating conditions, low cost, and good cycling stability. In this context, the study of mixed cobalt and nickel oxides with the general formula Co3-x Ni x O4 (0 ≤ x ≤ 1) was carried out, where four mixed metal oxides Co2.75Ni0.25O4, Co2.5Ni0.5O4, Co2.25Ni0.75O4, and Co2NiO4 have been successfully synthesized through the sol-gel method modified Pechini route. The structural investigation demonstrated that pure spinel structures were obtained for 0 ≤ x ≤ 0.75. A deep study was carried out with the main goal of finding the best phase that provides low redox temperature. Interesting reduction temperatures for all the compositions have been found, and the lowest values of 675 and 710 °C have been reported for Co2.25Ni0.75O4 and Co2.5Ni0.5O4, respectively. The thermal cycling results of this latest material using TGA measurement have proven attractive cycling stability of which the complete reoxidation of the samples was achieved. In addition, thermodynamic analysis of a reduction step was performed and good agreement of the theoretical reduction temperature of Co2.25Ni0.75O4 with the experimental one has been found.

Potential of solar thermochemical water-splitting cycles: A review

Hydrogen fuel is a valuable tool to achieve the energy transition process, and according to the 2050 net zero emissions scenario its demand is expected to increase by more than 530 Mt H2. This article discusses several routes available to produce hydrogen fuel, with a special focus on solar thermochemical cycles for Water Splitting (WS). Solar thermochemical WS cycles are a potential technology to produce green hydrogen and CO in the future due to their great potential to become a commercial scale process. This technology is still under development and some challenges related to components and sub-processes are being addressed by several research groups in order to make green hydrogen production by this route technically and economically feasible. Specific technological aspects such as particle heating methods, suitability and properties of redox materials for two-step cycles, methods to achieve low oxygen pressure in the reduction chamber, as well as the importance of implementing heat recovery are analysed in detail, as their performance have a significant effect on the overall efficiency and economic of the whole process. Current progress indicates that the realisation of a sufficiently efficient thermochemical cycle is possible within the few years, if the mentioned limitations are overcome.

Dynamic behavior of solar thermochemical reactors for fuel generation: Modeling and control strategies

Solar thermochemical water splitting provides an efficient route for converting solar energy into renewable fuels. The operation of solar thermochemical reactor under real on-sun condition requires in-depth understanding of its dynamic behaivor under fluctuating direct normal irradiation (DNI). In this work, we developed a 2D axial-symmetric multi-physical model to assess the reactor’s dynamic behavior with three oxygen removing technologies electrochemical oxygen pump (EOP), sweep gas (SG), and vacuum pump (VP), under varying solar irradiations. To alleviate the negative effect of decreased DNI, two control strategies (CS) including varying operational conditions for oxygen removing technologies (CS1) and replacing the reduction step with the oxidation step (CS2) were conducted. We found that CS1 was only effective when DNI reduction was larger than 75 % over the whole reduction step (case 2). In general, CS2 can be implemented to alleviate the efficiency decrease for all DNI variations expect for the case 2 in this study. Especially, CS2 significantly increased the solar-to-fuel efficiency compared to the case without control strategy for the reduced DNI from 1000 W/m2 to 500 W/m2 during the whole reduction step. The reactor dynamic behavior in response to the daily DNI variations is also reported which shows that the efficiency profiles are highly in line with the DNI profile. This modeling work as well as proposed control strategies can be used to guide the design and control of solar thermochemical reactors under realistic conditions and hence for better assessing the performance of the reactor in practice.

Study of a new receiver-reactor cavity system with multiple mobile redox units for solar thermochemical water splitting

Solar thermochemical redox cycles could be a path to efficient, large-scale renewable hydrogen production. A new receiver-reactor concept is presented that combines characteristics of the most successful receiver-reactor systems to date, with features of concepts showing the highest efficiency potential. The key features of the system are movable reactive structures in combination with linear transportation systems and dedicated oxidation reactors. The application of several of these units allows the continuous operation of the receiver-reactor and permits the implementation of a solid–solid heat recovery system. Both of these characteristics are important to increase the system efficiency beyond the current state of technology. A numerical model is developed to simulate a basic cubic design version of the new concept and to analyse its performance. Parameter variations are studied amongst others for different cavity sizes, solar concentration factors and numbers of movable reactive structures. By avoiding the cyclic heating of the inert reactor vessel, the model predicts high efficiency values above 14% even for non-optimized designs. Furthermore, the basic concept of the heat recovery system is modelled with heat recovery rates of up to 20% in its most simple implementation. The new concept has the features required for highly performant systems and opens up a large parameter space for reactor design and operation optimization.

Thermodynamic Guiding Principles for Designing Nonstoichiometric Redox Materials for Solar Thermochemical Fuel Production: Ceria, Perovskites, and Beyond

Two-step solar thermochemical water splitting is a promising pathway for renewable fuel production. Among the state-of-the-art materials, ceria tends to favor the oxidation step while perovskites favor the reduction step and developing a champion material with well-balanced redox thermodynamics remains a key challenge. A generic thermodynamic framework is thus presented to guide material design. More details can be found in article number 2000925, Wojciech Lipiński and co-workers.

Effects of coexisting oxoanions on SO<sub>3</sub> decomposition activity of molten-phase potassium metavanadate catalysts

Molten-state potassium metavanadate (KVO3) supported on mesoporous SiO2 materials have emerged as active catalysts for SO3 decomposition over a moderate temperature range (≤650 °C), which is a potential O2 evolution reaction useful for solar thermochemical water splitting. The molten phase formed at ≥520 °C contained tetrahedral VO42−, which plays a vital role in accelerating the SO3 uptake and conversion to SO2/O2. The present study aimed to reveal how the SO3 decomposition activity is affected by adding other oxoanions such as borate (BO33−), carbonate (CO32−), and phosphate (PO43−) into the melt. Although borate showed a deteriorating effect, phosphate tended to improve the catalytic activity when the P/V molar ratio was equal to or less than 0.5. The addition of phosphate produced a mixed phosphate vanadate with a composition of KV2PO8, which consists of infinite tetrahedral PO4 and pyramidal VO5 linked by vertex sharing. Because of the congruent melting at temperature as low as ∼530 °C, KV2PO8 may be expected as another candidate of active molten phase catalyst for SO3 decomposition.

The role of entropy in the success of nonstoichiometric oxides for two-step thermochemical water and CO2 splitting

Owing to their high theoretical efficiency, two-step solar thermochemical water and CO2 splitting cycles, using a metal oxide intermediate, provide a promising pathway to store solar energy in a stable chemical bond. To date, the most successful demonstrations have utilized nonstoichiometric oxides, a class of materials capable of continuously transitioning from an oxidized to reduced state via formation of oxygen vacancies. The success of nonstoichiometric oxides is typically attributed to their ability to maintain their crystallographic phase, allowing them to be cycled many times without destabilizing–a tremendous practical advantage. In this work, we utilize a combined empirical and statistical thermodynamics approach to present an alternative explanation of the success of nonstoichiometric oxides from an entropy perspective. We first illuminate the importance of the entropy change of the reduction reaction as a key material parameter. We then develop a methodology for plotting nonstoichiometric oxides on the Ellingham diagram, enabling a direct comparison to stoichiometric materials. This analysis reveals the unique ability of nonstoichiometric oxides to achieve a significant solid-state entropy contribution, which results from the disorder generated via oxygen vacancy formation. To quantify the solid-state entropy, we develop a simple configurational entropy model applicable to any oxygen-deficient nonstoichiometric oxide. We compare model predictions to existing data for two important nonstoichiometric oxides, CeO2−δ and La0.6Ca0.4Mn0.6Al0.4O3−δ, to reveal the main trends in entropy vs δ and material composition and discuss the causes and implications of deviations from the theory.

(Invited) An Overview of the U.S. Department of Energy's Consortia Focused on Enabling Low-Cost Hydrogen Production Via Water Splitting Technologies

Hydrogen provides a unique opportunity for affordable, reliable, clean, and secure energy across sectors, many of which can be hard to decarbonize. To enable low-cost and sustainable hydrogen production at scale, the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office (HFTO) supports RD&D to address critical challenges and barriers across a broad portfolio of hydrogen production technologies. One very promising pathway for large-scale hydrogen production is water splitting.Water splitting technologies range from commercial technologies such as electrolyzers, which are currently manufactured at the MW scale, to approaches that are at a much earlier stage of development, such as photoelectrochemical (PEC) and solar thermochemical (STCH) processes. All these water splitting pathways offer diverse benefits in energy storage, grid services, and cross-sector emissions reductions while taking advantage of our abundant renewable, nuclear, or fossil energy resources. However, as with most innovative and potentially high-impact clean energy technologies, critical materials-, component- and system-level challenges must be addressed to improve efficiency and durability, as well as reduce cost. As such, HFTO initiated the HydroGEN Advanced Water Splitting Materials (AWSM) Consortium in 2017 and recently launched the H2 from the Next-generation of Electrolyzers of Water (H2NEW) consortium.HydroGEN (https://www.h2awsm.org/), one of DOE’s 7 Energy Materials Network (EMN) consortia is dedicated to accelerating the materials R&D of low TRL advanced water splitting (AWS) technologies, is now in its second phase. The consortium comprises five core national laboratories and focuses on four early stage AWS pathways: alkaline exchange membrane (AEM) electrolysis, proton conducting high temperature electrolysis, photoelectrochemical, and solar thermochemical water splitting. Providing streamlined access to world-class expertise and experimental and computational capabilities, the consortium is currently supporting 23 university, industry and national laboratory R&D projects seeking to discover and design new materials to increase efficiency and durability of water-splitting systems. In addition, HydroGEN has recently initiated consortium-led efforts focused on the most critical barriers to these AWS pathways.With PEM electrolyzers already commercial and significant advancements in oxygen conducting solid oxide electrolysis cells (O-SOECs) realized, the critical R&D needs for these technologies are no longer at the foundational, materials development level. Yet these systems are still too expensive for wide-scale commercialization. To enable high-volume manufacturing of affordable, durable, efficient electrolyzers, H2NEW, another multi-lab consortium, was established in 2020. This comprehensive, concerted effort is focused on overcoming barriers related to components and materials integration, with an initial focus on durability, to achieve <$2/kg H2.

Uniting Experiment and Computational Modeling to Elucidate Structure-Property Relationships of Water Splitting Materials

The layered perovskite BaCe0.25Mn0.75O3 (BCM) is a proven solar thermochemical water splitting material demonstrating higher hydrogen generation and improved redox kinetics relative to current state of the art materials. However, fundamental understanding of the underlying mechanisms of BCM’s efficacy is necessary for solar thermochemical water splitting in related perovskite-type materials to become a viable pathway for green hydrogen generation. To this end, a family of BXM (X = Ce, Nb, Pr) materials has been synthesized and undergone in-depth, multi-faceted characterization. Despite the three materials having identical structures, and differing only in 1/8 of their cation species, their water splitting performances are markedly different. X-ray absorption spectroscopy (XAS) analysis reveals substantial differences in the electronic structure of oxygen between the three samples, both when fully oxidized and at quantified degrees of reduction. However, interpreting the changes in XAS spectra of these complex materials via conventional means has proven to be challenging. The Molecular Foundry has successfully simulated the XAS spectra of the oxidized and defected structures allowing specific peaks in the XAS spectra to be correlated to molecular orbitals and providing invaluable insights into the underlying electronic changes observed experimentally. Additionally, both in situ and ex situ electron energy loss spectroscopy (EELS) analysis of the BXM materials allowed changes in the O K-edge and the Mn L-edge to be monitored with increasing extent of reduction and directly compared to XAS. The unique advantage of performing STEM-EELS analysis of these materials during in situ thermal reduction is the ability to observe underlying structural modifications, such as phase changes or defect formations, in tandem with the electronic changes in the material. Taken together, the results reported here provide a key component towards elucidating structure-property relationships for this class of solar thermochemical hydrogen production (STCH) materials.

Hydrogen from Sunlight and Water: A Side-by-Side Comparison between Photoelectrochemical and Solar Thermochemical Water-Splitting

Hydrogen from Sunlight and Water: A Side-by-Side Comparison between Photoelectrochemical and Solar Thermochemical Water-Splitting

Simulation-based environmental-impact assessment of glycerol-to-hydrogen conversion technologies

Abstract This simulation-based comparative assessment aims to quantify the environmental and human-health impacts of greener hydrogen (H2) production via three glycerol-based technologies, including: supercritical water reforming (SCWR), aqueous-phase reforming (APR) and autothermal reforming (ATR). The GaBi (2018 edition) life-cycle assessment (LCA) platform is used to develop cradle-to-gate product system models for these technologies and the TRACI 2.1 methodology is used to quantify their midpoint impact categories. Aspen HYSYS (v11) process-simulation software is used to generate the life-cycle inventory (LCI) primary data required to produce 1 kg of H2 via each of the indicated glycerol-reforming technologies. Per ISO 14040:2006 reporting requirements for the LCA results interpretation step, three base case (BC) scenarios and four sensitivity scenarios (SS) are developed and quantified to compare the effects of different process electricity sources (US grid mix versus wind power) and thermal energy sources (natural gas versus biogas) on the LCA results. The high operating pressure (viz. 240 bar) of SCWR enabled assessment of the impact of in situ electricity generation to offset some of electricity required for this technology. The major insights from this research are as follows: (i) per 1 kg of produced H2, APR reduces CO2 emissions by ≈95% compared to ATR and by ≈92% compared to SCWR, (ii) for BC scenarios, the primary energy consumption (in MJ/kg of produced H2) is in the following order from highest to lowest: ATR &amp;gt; SCWR &amp;gt; APR and (iii) H2 production via glycerol APR is more environmentally sustainable than SCWR and ATR, and thus offers a promising path for greener H2 production. Future environmental sustainability studies should focus on expanding the scope of this study to include H2 production via water electrolysis using renewable electricity sources and via solar and nuclear-driven thermochemical water splitting.

Hydrogen Production by Solar Thermochemical Water-Splitting Cycle via a Beam Down Concentrator

Hydrogen Production by Solar Thermochemical Water-Splitting Cycle via a Beam Down Concentrator

White is the color of hydrogen from concentrated solar energy and thermochemical water splitting cycles

White is the color of hydrogen from concentrated solar energy and thermochemical water splitting cycles

Thermodynamic Guiding Principles for Designing Nonstoichiometric Redox Materials for Solar Thermochemical Fuel Production: Ceria, Perovskites, and Beyond

Two‐step solar thermochemical water splitting is a promising pathway for renewable fuel production due to its potential for high thermal efficiency via full‐spectrum sunlight utilization. Such a promise critically relies on simultaneous innovation in the redox materials and the reactor systems. Most prior efforts on material design are focused on improving the fuel yield at lower reduction temperatures. However, developing materials with both high fuel output and efficiency remains a key challenge, requiring a rigorous understanding of the effects of material thermodynamic properties. Herein, a generic thermodynamic framework is described to decipher the material effects by studying both the state‐of‐the‐art and hypothetical materials within a counterflow reactor system. A global efficiency map is presented for redox materials, revealing inevitable tradeoffs among competing factors such as thermal losses, sweep gas and oxidizer demand, solid preheating, and reduction enthalpy. The choice of the most efficient material is closely linked to the system conditions. Ceria‐based materials outperform perovskites under most scenarios, and the optimal hypothetical materials tend to favor higher reduction enthalpies and entropies than existing materials. This work offers a valuable material design roadmap to identify solutions toward efficient solar fuel production.